The AlGaInP alloy system has been used for making high quality semiconductor lasers with an emitting wavelength around 670 nanometers. This alloy system may also be useful for making light-emitting diodes (LEDs) in the wavelength range from about 560 to 680 nanometers by adjusting the aluminum to gallium ratio in the active region of the device. Increasing aluminum proportion provides shorter wavelengths. It has also been demonstrated that organometallic vapor phase epitaxy provides means for growing optically efficient AlGaInP heterostructure devices.
In a surface-emitting LED of the type used for the vast majority of LED displays, the device geometry is simple, as shown in FIG. 1. In such a device, there is a GaAs n-type absorbing substrate 10 on which several epitaxial layers are grown to form the LED. First, there is an n-type confining layer of AlGaInP epitaxially grown on the GaAs substrate. An active layer 12 of AlGaInP with an aluminum-gallium ratio selected to provide a desired wavelength of emission is epitaxially grown on the n-type confining layer. The active layer is topped by a p-type confining layer 13, also of AlGaInP. A front electrical contact 14 is provided on the front or emitting face of the LED. A back contact 15 comprises a layer of metal clear across the GaAs substrate. Typically in use, the back electrical contact is mounted on and electrically connected to a metal lead.
Efficient operation of the LED depends on current injected from the metal front contact 14 spreading out laterally to the edges of the LED chip so that light is generated uniformly across the p-n junction. If the sheet resistance of the top layer of semiconductor, namely, the upper p-type confining layer 13, is not low enough, the current does not spread sufficiently and tends to flow directly down beneath the front contact toward the back contact. This type of "current crowding" results in most of the light being generated under the opaque front contact. Much of the light generated in this region is blocked and absorbed within the LED chip, thereby severely limiting the efficiency of the device.
In the usual case of an LED grown with the p-type AlGaInP nearer the top or front surface of the LED, the front layer has a very high resistivity and lateral current flow from the contact is severely restricted. The high resistivity is a result of limited p-type dopant levels that are achievable in AlGaInP and the low hole mobility in such material. Further, resistivity increases as the proportion of aluminum is increased in the alloy, so the problem becomes more acute for short-wavelength LEDs.
The techniques proposed for minimizing the current crowding problem in AlGaInP LEDs have not been completely satisfactory. One technique is to employ patterns of the front metal contact which mechanically aid in spreading the current. Such patterns comprise fingers or grid lines which extend away from the bonding pad to which a wire bond is made. Such techniques are commonly used in LEDs as well as other devices where current spreading is desired. Even so, most of the light generated is under the opaque metal patterns and is blocked. Another technique is to use a transparent front electrical contact such as indium-tin oxide instead of metal. Such transparent electrical contacts have high resistivity and lead to high series resistance in the device.
Because of such shortcomings, it is desirable to provide a technique for distributing current from the front contact to the active p-n junction so that light is emitted more uniformly throughout the junction and device efficiency is enhanced.